The present invention relates to a steel for steel sheets excellent in workability, and a method of deoxidizing the steel. The present invention particularly relates to a steel for easily workable steel sheets which give sheet products having remarkably decreased surface defects, and a deoxidation method for preventing clogging of a casting nozzle during continuous-casting the molten steel.
Extra low carbon steels have been widely used in recent years for outer panels of automobiles, surface-treated steel sheets, etc. because the steels have excellent workability. The slabs for steel sheets are usually produced by decarburizing molten steel produced by a converter, etc., using a vacuum degassing procedure such as the RH process to lower the carbon concentration to an extra low carbon concentration, and continuous casting. The molten steel subsequent to decarburization must be deoxidized before continuous casting because excessive free oxygen, at a high concentration, is present therein.
In general, aluminum (Al) is used for deoxidizing the molten steel, and the conventional extra low carbon steel is an Al-killed steel. Alumina inclusions formed by deoxidizing the molten steel form clusters, which often remain in the surface layer of a continuous-cast slab to cause formation of surface defects subsequent to rolling and which remarkably lower the yield of the products. When an extra low carbon steel is to be produced, it is therefore important, from the standpoint of ensuring the surface quality, to take measures to clean the molten steel.
On the other hand, it is known that nozzle clogging taking place during continuous casting causes formation of seams such as scabs and slivers, and markedly lowers the quality of product steel sheet materials. Moreover, it is also necessary to take measures to prevent the nozzle clogging from the standpoint of ensuring high productivity which is the merit of continuous casting.
It is known that formation of nozzle clogging is caused by adhesion of alumina and alumina clusters present in the molten steel to, and the accumulation of these materials on, the inner wall surface of the immersion nozzle. Various theories about the mechanism nozzle clogging are explained in detail in xe2x80x9cRefractories,xe2x80x9d vol. 46 (1994) pages 166-178. It is known from observation of the cross-sections of clogged nozzles that the material adhering to the inner wall surfaces of the nozzles is a mixture of alumina clusters and metal. Accordingly, in order to prevent the nozzle clogging, increasing the cleanliness of the molten steel is most important.
Measures to improve the cleanliness of the molten steel are explained in detail in xe2x80x9cHighly Clean Steel,xe2x80x9d in the 126th and 127th Nishiyama Kinen Gijyutsu Koza (126th and 127th Nishiyama Memorial Technological Lectures) (Edited by Japan Iron and Steel Association) pages 12-14. For example, the following is described. Prevention of reoxidation of molten steel in a tundish is important, and the reoxidation can be inhibited by sealing the atmosphere within the tundish with an inert gas.
Furthermore, for the purpose of preventing nozzle clogging, a method has been proposed wherein alumina, which causes nozzle clogging, is morphologically controlled to form composite inclusions having a low melting point, and the inclusions hardly adhere to the inner wall surface of the nozzle. It is addition of Ca in molten steel that has heretofore been applied as a method of lowering the melting point of the inclusions. This effect of preventing the nozzle clogging is described in xe2x80x9cIron and Steel,xe2x80x9d (1986) S281. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 5-237613 discloses a method of adding Ca to within a tundish in accordance with the state of nozzle clogging.
However, in order to lower the melting point of alumina clusters in the Al-killed steel, a large amount of Ca must be added, and the effect of preventing the nozzle clogging is not necessarily pronounced. As a result, there arises the problem that stabilized production of steel sheet products having no surface defects is difficult when the Al-killed steel is used.
The present inventors have previously shown (e.g., in Japanese Unexamined Patent Publication (Kokai) No. 7-111071) that a Ti-containing steel containing from 0.01 to 0.40 wt % of Ti and a decreased amount of Al has properties comparable to those of an Al-deoxidized steel, as a sheet material, and that the surface defects of a steel sheet produced from the steel can be decreased.
The reason why the Ti-containing steel containing Al in a decreased amount hardly forms surface defects is thought to be that inclusions in the molten steel are not Al2O3 inclusions which tend to coalesce to form large clusters but ones mainly containing Ti oxide.
According to the knowledge of the present inventors, however, such a Ti-containing steel is likely to clog a teeming nozzle, compared with the Al-deoxidized steel. Accordingly, in order to produce a Ti-containing extra low carbon steel sheet material having no surface defects by continuous casting, establishment of a technology for preventing clogging of a teeming nozzle is indispensable.
The present inventors have therefore proposed, as methods of preventing a Ti-containing steel containing a decreased amount of Al from clogging the nozzle, a method wherein a nozzle prepared from a specific material is used (Japanese Unexamined Patent Publication (Kokai) No. 7-111071), a method wherein the Si concentration in the molten steel is controlled (Japanese Unexamined Patent Publication (Kokai) No. 7-111073), a method wherein the oxygen concentration in the molten steel is controlled (Japanese Unexamined Patent Publication (Kokai) No. 7-111072), and the like methods.
However, the methods of controlling the concentration of Si or O cannot be applied to some steels because the methods influence the steel materials. Moreover, it has been difficult for each of the methods to completely prevent the Ti-containing steels from clogging the nozzle. It has therefore been desired to provide means which prevents the nozzle clogging more surely, and which can stably produce extra low carbon steel sheet products containing Ti without surface defects.
Accordingly, an object of the present invention is to surely prevent the above-mentioned Ti-containing steel, being likely to clog the casting nozzle, from clogging the nozzle without exerting adverse effects on the product quality and to provide a steel for Ti-containing extra low carbon steel sheets capable of significantly decreasing formation of surface defects of the products.
Inclusions tend to adhere to the inner surface of the nozzle and grow when the inclusions have a high melting point and show a significant coalescing tendency. Accordingly, another object of the present invention is to provide a deoxidation method capable of radically preventing the nozzle clogging by controlling the chemical composition of the inclusions in the molten steel so that inclusions having a low melting point and showing a decreased coalescing tendency are formed.
The present inventors have made the following discovery. Although addition of Ca in a Ti-containing steel forms inclusions having a low melting point, and decreases the surface defects of a steel sheet material, the addition amount of Ca must be adjusted so that the total Ca concentration in the molten steel falls into a predetermined range because the effect of lowering the melting point is nullified when the Ca addition amount is increased.
Furthermore, pre-deoxidation of the molten steel with a small amount of Al is desirable from the standpoint of stabilizing a high yield of Ti. However, excessive addition of Al forms inclusions similar to those in an Al-deoxidized steel, and there arises the problem that inclusions having a low melting point are not formed unless a large amount of Ca is added, as explained above. Accordingly, the present inventors have discovered that when pre-deoxidation with Al is conducted, proper control of the conditions for the pre-deoxidation with Al prior to Ti addition and adjustment of a Ca addition amount as explained above are required.
Although pre-deoxidation with Si prior to Al deoxidation is possible, Si must be used within the permissible level of a [Si] concentration (0.03%) in the molten steel because CaSi is generally used as a low cost Ca source.
Furthermore, the present inventors have made discoveries related to a proper chemical composition of the steel for Ti-containing extra low carbon steel sheets having decreased surface defects and a proper chemical composition of inclusions in the steel from the discoveries mentioned above and from the results of examining the product material.
The present invention has been achieved based on these discoveries. The steel for steel sheets according to the present invention is one for steel sheets excellent in workability, comprising, based on weight, 0.0001 to 0.0030% of C, up to 0.03% of Si, 0.05 to 0.30% of Mn, up to 0.015% of P, 0.001 to 0.015% of S, up to 0.008% of Al, 0.02 to 0.08% of Ti, 0.0005 to 0.0020% of Ca, 0.0005 to 0.01% of N and the balance of Fe and unavoidable impurities.
The steel can also comprise at least one of 0.001 to 0.02 wt % of Nb and 0.0001 to 0.0010 wt % of B.
The present invention also provides a steel for steel sheets excellent in workability and having the chemical composition of the steel mentioned above, wherein at least 70% of nonmetallic inclusions having a circle-equivalent diameter of at least 10 xcexcm have a chemical composition with regard to the three components, CaO, Al2O3 and TiO2, falling into the range represented by the following formulas (1a) to (1c):
0.03xe2x89xa6CaO/(CaO+Al2O3+TiO2)xe2x89xa60.30xe2x80x83xe2x80x83(1a)
0xe2x89xa6Al2O3/(CaO+Al2O3+TiO2)xe2x89xa60.40xe2x80x83xe2x80x83(1b)
0.40xe2x89xa6TiO2/(CaO+Al2O3+TiO2)xe2x89xa60.90xe2x80x83xe2x80x83(1c)
where CaO, Al2O3 and TiO2 represent the contents (wt %) of CaO, Al2O3 and TiO2, respectively, in the nonmetallic inclusions.
In addition, Ti oxide in the actual cast slab sometimes has the forms of TiO2 and Ti2O3, the Ti oxide is represented herein by TiO2 for convenience.
The gist of a method of deoxidizing a steel for steel sheets excellent in workability according to the present invention is as described below.
(1) A method of deoxidizing a steel for steel sheets excellent in workability, wherein molten steel to be continuous-cast to give any of the steels mentioned above is deoxidized, comprising adding a predetermined amount of Ti or a Ti alloy to the non-deoxidized molten steel, and then adding Ca or a Ca alloy thereto to allow a total Ca concentration in the steel to fall into 5 to 20 ppm, whereby the nonmetallic inclusions in the molten steel are formed with Ti oxide containing CaO.
(2) A method of deoxidizing a steel for steel sheets excellent in workability, wherein molten steel to be continuous-cast to give any of the steels mentioned above is deoxidized, comprising pre-deoxidizing the molten steel with Al to allow a total Al concentration to fall into 10 to 80 ppm, adding a predetermined amount of Ti or a Ti alloy, and further adding Ca or a Ca alloy thereto to allow a total Ca concentration in the molten steel to fall into 5 to 20 ppm, whereby the nonmetallic inclusions in the molten steel are formed with Ti oxide containing CaO and Al2O3.
In addition, the total Al concentration is a weight ratio of Al in the molten steel plus Al in the inclusions to the molten steel, and the total Ca concentration is also similarly defined.
(3) The method of deoxidizing a steel for steel sheets excellent in workability described in (2) mentioned above, wherein the free oxygen concentration in the non-deoxidized molten steel is measured, and the Al addition amount in the pre-deoxidation is allowed to fall into the range represented by the formula
WAl less than 162.5+0.375 C. (O)xe2x80x83xe2x80x83(2)
wherein WAl is an addition amount of Al per ton of the molten steel (g/t), and C (O) is a free oxygen concentration (ppm) in the non-deoxidized molten steel.
(4) The method of deoxidizing a steel for steel sheets excellent in workability according to any of (1) to (3) described above, wherein at least 70% of the nonmetallic inclusions in the steel having a circle-equivalent diameter of at least 10 xcexcm have a chemical composition with regard to the three components, CaO, Al2O3 and TiO2, falling into the range represented by the formulas (1a) to (1c).